Mexico's improved train service bids for US tourists. When you get hungry, you may be approached by a girl selling jicama

If the idea of train travel in Mexico conjures up scenes from John Huston's classic film ``The Treasure of the Sierra Madre'' or images of journalist John Reed riding with Mexican revolutionaries, you're just a bit behind the times. The government train system has undergone an overhaul in the last two years. Several routes, including trips from the border to Mexico City, have been completely renovated. From the Texas-Mexico border it's possible to take the top-of-the-line primera especial service at Piedras Negras, Nuevo Laredo, Reynosa, or Matamoros. A porter in a white jacket with silver buttons brings clean towels and prepares your Pullman bed. Waiters serve full-course meals with a choice of steak, chicken, or fish, along with a drink, in the comfortable dining car. All of this is included in the fare, which costs less than one-third of the price of a one-way airline ticket.

With the recent bankruptcy of Aeromexico, one of Mexico's two airlines, as well as rising air fares, train travel is well worth considering, especially if you want the adventure to begin the minute you cross the border. The Pacific route from Mexicali offers the ambiance of an original Pullman car, with a proximity to local color that is missing in the air-filtered sterility of a plane. This 46-hour, 2,758-kilometer (1,700-mile) trek provides a more leisurely introduction to Mexico.

About $78 pays for a dormitorio, a 3-by-6-foot Pullman compartment with a bed that pulls out of the wall, two green upholstered seats, a window half the length of the compartment, and a small stainless steel sink in a corner.

The ticket includes two meals on the last 12-hour leg of the journey, from Guadalajara to Mexico City. By that time you should be fairly ravenous, since there is no dining car or club car for the first 34 hours. Air conditioning is another luxury you will have to do without. Before boarding the train, buy a couple gallons of frozen water at the station. You're going to need it.

As the train passes through and stops at seafood towns like Puerto Penasco and dusty depots such as La Trinchera, local vendors shout, ``Tacos dorados, paletas!'' from the platform. They charge about 2,000 pesos (less than $1) for the potato-and-bean tacos in plastic sandwich bags and even less for the paletas, tamarind ``Popsicles'' that temporarily quench your thirst.

During a typical trip, an eight-year-old girl with long dark hair might board the train at Acaponeta selling jicama (vine root) and mangoes seasoned with chili. If you don't sample her wares, she gives you a reproachful look with her greenish-brown eyes and tells you she is an orphan.

The sparse, cactus-studded landscape of the Sonora resembles a western movie as Mexican cowboys with wide-brimmed sombreros sit solemnly on horses or burros. From abandoned railroad cars that have been dressed up with kitchen curtains, locals enthusiastically wave to you, while chickens and dogs run in dusty circles.

The town of Benjamin Hill (pop., 8,000), where the train stops for about 40 minutes, is worth exploring. The air-conditioned botica (pharmacy) on the unpaved main street sells toallitas humedas (like Wet Wipes) that will be very useful throughout the trip. Ask the friendly male clerk for directions to the statue of the town's namesake, a revolutionary general.

If you're tired enough to ignore the pervasive heat of the Sonora Desert, the train's motion will lull you to sleep, and you'll awaken on the second morning to a cool sea breeze and the sound of crashing surf. About 9 a.m. the train pulls into sun-drenched Mazatl'an, the largest Mexican port on the Pacific coast. Vendors line the sidewalk selling unshelled shrimps with lime, or roast meat, whole fish, and chickens over a spit with green onions, while keeping an eye on iron vats filled with bubbling black beans.

As it heats up later in the day, you may want to exchange the privacy of your own personal sauna for some fresh air in the vestibule, and a chance to meet fellow travelers. On the Pacific route there are few American tourists. You'll meet people like 21-year-old Eduardo Rodr'iquez, a Mexican who slipped across the border into the United States and worked in a restaurant in Oxnard, Calif., until he received a traffic ticket. Anxious because of his illegal status, he decided to return to Mexico rather than go to court.

The landscape becomes progressively greener as the train winds away from the coast and passes fields of corn and groves of apricots and mangoes. Strings of long, thin yellowing chilies dry in the sun and the domes of colonial churches stand out among multicolored houses.

Arturo Serratos, the reserved porter, who has worked for the railroad 25 years, excitedly tells passengers they will change to a renovated train in Guadalajara, with a restaurant, air conditioning that works, and a club car.

Arturo does not exaggerate. The new train is so well air conditioned that the one blanket provided does not suffice. The Pullman compartment is nearly identical in design, but much cleaner, with new burgundy upholstery and a tiny closet for hanging business suits. Mexican businessmen and freshly made-up senoras in silky dresses and high heels fill the dining car as white-jacketed waiters serve the long-awaited meals.

The next morning, two days away from the US border, the train slowly pulls into Mexico City as passengers finish their breakfasts. Businessmen arrive ready to work and North American tourists have already eased into the slower pace of Mexican life. Practical information

Reservations should be made at least two weeks in advance through Romero's Mexico Service in Newport Beach, Calif. (714) 548-8931. If you want to stop along the way, perhaps in the popular beach town of Mazatl'an, book the ticket from the border to your chosen spot and purchase the second ticket upon arrival.

For further informationcontact the Mexican Government Tourist Office, 405 Park Ave., Suite 1002, New York, NY 10022. Tel. (212) 755-7261.